1. Field of the Invention
The present invention relates to a composite anode active material, a method of preparing the same, and an anode and lithium battery containing the composite anode active material, and more particularly, to a composite anode active material including an intermetallic compound, a transition metal and a carbon composite, a method of preparing the same, and an anode and lithium battery containing the composite anode active material.
2. Description of the Related Art
In an effort to achieve high voltages and energy densities, research and development has been extensively conducted into non-aqueous electrolyte secondary batteries using lithium compounds as anodes. Specifically, metallic lithium has become the subject of intense research due to its ability to impart high initial battery capacity. However, when metallic lithium is used as an anode material, large amount of lithium can deposit on the surface of the anode in the form of dendrites, which may degrade the battery charge and discharge efficiency or cause internal-shorts between the anode and the cathode. Furthermore, lithium is sensitive to heat and impact, and is prone to explosion due to its instability and high reactivity. These problems have tended to limit the commercialization of batteries with metallic lithium. In order to eliminate these problems with the use of metallic lithium, carbonaceous materials have been proposed for use as anode materials. Carbonaceous anodes aid in redox reactions such that lithium ions in an electrolytic solution intercalate/deintercalate in the crystal lattice structure of the carbonaceous material during the charge and discharge cycles. These anodes are referred to as a “rocking chair” type of anodes.
The carbonaceous anode has contributed to the use of lithium batteries by overcoming various disadvantages associated with metallic lithium. However, electronic equipment is becoming smaller and lighter in weight, and the use of portable electronic instruments is becoming more widespread, making the further development of lithium secondary batteries having higher capacities of interest.
Lithium batteries using carbonaceous anodes have low battery capacities because of the porosity of the carbonaceous anodes. For example, graphite, which is a highly crystalline material, when made into a structure in a form of LiC6 by reacting with lithium ions, has a theoretical specific capacity of about 372 mAh/g. This is only about 10% that of metallic lithium, which has a capacity of about 3860 mAh/g. Thus, in spite of many problems with conventional metallic anodes, studies for improving battery capacity using metallic lithium as an anode material are being carried out.
A representative example of such studies is the use of materials that can alloy with lithium, e.g., Si, Sn, Al, or the like, as anode active materials. However, materials that can alloy with lithium, such as Si or Sn, may present several problems, including volumetric expansion during formation of the lithium alloy, creation of electrically disconnected active materials in an electrode, aggravation of electrolytic decomposition due to increases in surface area, and so on.
In order to overcome these problems with the use of such a metallic material, a technique of using a metal oxide exhibiting a relatively low volumetric expansion as an anode active material has been proposed. For example, use of an amorphous Sn-based oxide has been proposed which minimizes the Sn particle size and prevents agglomeration of Sn particles during charge and discharge cycles, thereby leading to improvement of capacity retention characteristics. However, Sn-based oxides unavoidably cause reactions between lithium and oxygen atoms, which is responsible for considerable irreversible capacities.
An intermetallic compound may be utilized between Sn and Si, and Cu, Fe and Mg. The intermetallic compound minimizes the particle size of Sn and Si, and does not induce a reaction of forming Li2O due to oxygen absence, thereby improving initial efficiency. However, the intermetallic compound undergoes agglomeration because as the cycle number is increased, the sizes of Sn and Si are increased as compared to the initial stage, and thus capacity retention characteristics of the intermetallic compound are gradually degraded.